In air-conditioning equipment for a large building such as an office building and an apartment house, the required air cooling capacity or heating capacity differs depending on the number of operating indoor units. Therefore, to meet this condition, a plurality of outdoor units are sometimes used.
In this case, each of the outdoor units is provided with a compressor, a four-way valve (directional control valve), an outdoor heat exchanger, an outdoor expansion valve, and an accumulator, and the outdoor units are connected in parallel to refrigerant piping via branch pipes.
As the compressor, a variable-speed compressor (inverter compressor) in which the rotational speed thereof is variable due to inverter control or a constant-speed compressor in which the rotational speed is constant is usually used. Preferably, to keep the pressure difference between the discharge side and the suction side in a predetermined range, the compressor is provided with a hot gas bypass circuit, which includes a solenoid valve and an expansion mechanism arranged in series, between a discharge pipe and a suction pipe.
The outdoor unit is operated according to the capacity required on the indoor side, and therefore in some cases, for example, only one outdoor unit is operated, and other outdoor units are not operated (hereinafter, an outdoor unit not being operated is sometimes referred to as a “non-operating outdoor unit”).
In such a case, a refrigerant accumulates in the non-operating outdoor units, so that in the outdoor unit being operated, a shortage of refrigerant may occur. If the refrigerant runs short, the liquid-side piping becomes in a two-phase state of gas and liquid, and problems of the decreased capacity of indoor unit, production of refrigerant noise, and the like occur.
To solve these problems, in the invention described in Patent Document 1 (Japanese Patent Application Publication No. 2000-220894), when a shortage of refrigerant occurs in the outdoor unit being operated, the non-operating outdoor units are operated so as to supply the refrigerant accumulating in the non-operating outdoor units to the refrigerant piping.
According to the invention described in Patent Document 1, the refrigerant can be supplied quickly to the outdoor unit being operated, in which the refrigerant runs short. However, this invention is unpreferable in terms of energy saving because electric power necessary for starting the compressors of the non-operating outdoor units is consumed.